In a typical optical WDM distribution system, a plurality of multiplexed signals are propagated over an optical medium to each receiver in the system and one of these signals is selected for detection within the receiver. The selection is often accomplished by interposing a wavelength sensitive optical filter between the medium and each receiver. In more specific terms, for instance, optical signals from each of N different optical signal generators with wavelengths of .phi..sub.1, .phi..sub.2, . . . , .phi..sub.N, respectively, are multiplexed and propagated over a system fiber connecting the various receivers. A given filter passes one of the wavelengths, say .phi..sub.i, from the composite set of wavelengths present on the fiber through to the associated receiver. For enhanced performance, the filter may be tunable so that a particular wavelength from the prescribed set may be optionally seleted. Such a tunable channel selection technique is important in order to increase the information capacity and system flexibility of short-haul communication systems such as in the local telephone loop wherein the number of multiplexed signals can range into the hundreds.
Conventional WDM filtering techniques have utilized dielectric-coated, optical interference filters or diffraction gratings. Interference filters usually have limited spectral resolution and are not tunable, thereby restricting their use to systems with a few number of channels. Diffraction gratings offer better spectral resolution, but because they are relatively bulky, lousy and expensive, they are not viable for in-line, short-haul applications.
Several, more recent alternatives have been proposed. Representative of these alternatives to tunable channel selection, for instance, is the article "Tunable Optical Multi/Demultiplexer for Optical FDM Transmission System", authored by Inoue et al and published in the Electronics Letters, vol. 21, pp. 387-389 (1985). As reported, a Mach-Zender interferometer is proposed as a tunable optical filter with very fine spectral resolution, but the so-called finesse is only on the order of 2 and this severely limits the number of channels that may be filtered. Moreover, the configuration is rather complex in that two directional couplers, a piezoelectric phase shifter, and polarization-holding fibers are required. Another article, entitled "Electro-Optically Tunable, Narrowband Ti:LiNbO.sub.3 Wavelength Filter", by Heismann et al as published in the Electronic Letters, vol. 23, pp. 572-574 (1987), reports on a tunable filter, but it has limitations in that high operating voltages are required and the tuning range is narrow.
Also, different types of Fabry-Perot interferometers have been proposed as a tunable filter for optical channel selection. Representative of these proposals are the following two articles: "WavelengthSelective Filters for Single-Mode Fiber WDM Systems Using Fabry-Perot Interferometers" authored by S. R. Mallinson and appearing in Applied Optics, 26, pp. 430-436 (1987); and "Pigtailed High-Finesse Tunable Fiber Fabry-Perot Interferometers with Large, Medium and Small Free Spectral Ranges" authored by J. Stone et al and appearing in Electronics Letters, Vol. 23, pp. 781,783 (1987). These techniques involve difficult alignment procedures to achieve high parallelism of interferometer surfaces. Furthermore, optical wavelength tuning requires very accurate linear movement (on the order of much less than one micron) usually using a piezoelectric element, and very high thermal stability, especially in a real field environment.
In general, the recent alternatives are deficient for tunable channel selection, short-haul applications because of one or more of the following limitations: require highly accurate adjustment and positioning; reflection of unselected signals may introduce source and system noise; cannot be utilized inline; limited spectral resolution; small finesse; high voltages required; difficult alignment and tunability mechanism; complexity; bulkiness; lousy; and expensive.